Endogenous as well as augmented NK reactivities are being characterized with respect to the following: 1) genetic control of the phenomena which involves the study of endogenous and augmentable NK reactivities in parental, F1, F2 and backcross mice of differet genetic backgrounds, 2) determination of strain dependence versus target dependence by the use of different tumor target - mouse strain combinations, 3) separation (purification) of effecter cells by a number of biophysical criteria, e.g., size, adherence to nylon wool, G-10 sephadex, plastic, 4) serological characterization of NK population(s) by the use of specific anti-NK reagents, 5) sorting studies by the use of flow cytometry in a FACS-III system based on light scatter and serological markers.